1. Field of the Invention
This invention relates to the multiplex addressing of ferroelectric liquid crystal (FELC) displays.
2. Discussion of Prior Art
Such displays typically comprise a layer of a FELC material contained between two cell walls each carrying strip electrodes forming an x, y matrix of addressable elements or pixels, at electrode intersections.
One type of device is known as a surface stabilised FELC display; see for example Meyer, R B 1977 Molec. Crystals liq. Crystals 40, 33, and Clark, N A and Lagerwall, S T, 1980, Appl. Phys. Lett. 36, 899. It can be switched between two molecular orientations by a dc pulse of suitable amplitude, time, and sign. Conceptually the liquid crystal molecules can be considered as rotating around a conical surface as the material is switched.
One prior art addressing scheme uses a strobe pulse of duration two time slots (ts), and amplitude zero in the first time slot, Vs in the second time slot sequentially applied to each x row electrode in turn. Meantime one of two data waveforms are applied to each y column electrode. The data waveforms are alternative dc pulses of alternate polarity and equal magnitude (+Vd, -Vd) each pulse lasting 1ts; one data waveform is the inverse of the other. This is termed a mono pulse strobe addressing scheme.
Another addressing scheme; described in GB 2,232,802, uses a strobe waveform having two pulses each lasting 1ts in combination with data waveforms as in the mono pulse strobe scheme. The leading strobe pulse may be zero or non zero and of variable amplitude and sign. Combination of strobe and data (resultant waveform) provides two different shapes of resultant. This is useful in changing the switching characteristics of the liquid crystal material. The time taken to address each pixel in a row is the line address time (lat) and for the above scheme is 2ts.
A variation of the above is described in GB 2,262,831. In this the strobe is applied to each row in turn with a 2ts interval between applications of strobes to each new row, as in the previous scheme. Additionally the strobe waveform is extended into the addressing time of the next addressed row, ie for part of the time strobe waveforms are being applied to 2 rows at the same time.
Another addressing scheme uses 4ts to address each pixel in a time. The strobe is a zero for 1ts, then Vs for 3ts. Data waveforms are of amplitude -V.sub.d, +V.sub.d, +V.sub.d, -V.sub.d (or the inverse) in successive time slots.
All addressing schemes must switch the material when required the difference between schemes is their performance. Performance is defined with respect to voltage used (low is desired), speed of switching (fast is desired), operating range (wide difference between selected and non selected voltages), and low dependence on pixel pattern. A high contrast between the two switched states is also advantageous; as is a wide operating range in temperature.
As noted above molecules switch from one side to the other side of a cone (eg ideally switch between .+-.22.degree. to an alignment direction), due to the application of a dc voltage applying a switching torque on each molecule. This switching torque causes switching around the (imaginary) surface of a cone.
Previous addressing schemes have been empirical in nature, their design being based on the results of experimental observation. Consequently the prior art addressing schemes, and in particular the pulse shapes, have not been optimised.